CHAPTER 12 - PRINTED CIRCUIT BOARD (PCB) DESIGN ISSUES.pdf

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P RINTER C IRCUIT B OARD I SSUES
CHAPTER 12: PRINTED CIRCUIT
BOARD (PCB) DESIGN ISSUES
INTRODUCTION
12.1
SECTION 12.1: PARTITIONING
12.3
SECTION 12.2: TRACES
12.5
RESISTANCE OF CONDUCTORS
12.5
VOLTAGE DROP IN SIGNAL LEADS—"KELVIN FEEDBACK"
12.7
SIGNAL RETURN CURRENTS
12.7
GROUND NOISE AND GROUND LOOPS
12.9
GROUND ISOLATION TECHNIQUES
12.11
STATIC PCB EFFECTS
12.15
SAMPLE MINIDIP AND SOIC OP AMP PCB GUARD LAYOUTS
12.17
DYNAMIC PCB EFFECTS
12.19
INDUCTANCE
12.21
STRAY INDUCTANCE
12.21
MUTUAL INDUCTANCE
12.22
PARASITIC EFFECTS IN INDUCTORS
12.24
Q OR "QUALITY FACTORS"
12.25
DON'T OVERLOOK ANYTHING
12.26
STRAY CAPACITANCE
12.27
CAPACITATIVE NOISE AND FARADAY SHIELDS
12.28
BUFFERING ADCs AGAINST LOGIC NOISE
12.29
HIGH CIRCUIT IMPEDANCES ARE SUSCEPTIBLE TO NOISE
PICKUP
12.30
SKIN EFFECT
12.33
TRANSMISSION LINES
12.35
DESIGN PCBs THOUGHTFULLY
12.36
DESIGNNING+B46 CONTROLLED IMPEDANCE TRACES ON
PCBs
12.36
MICROSTRIP PCB TRANSMISSION LINES
12.38
SOME MICROSTRIP GUIDELINES
12.39
SYMMETRIC STRIPLINE PCB TRANSMISSION LINES
12.40
SOME PROS AND CONS OF EMBEDDING TRACES
12.42
DEALING WITH HIGH SPEED LOGIC
12.43
LOW VOLTAGE DIFFERENTIAL SIGNALLING (LVDS)
12.49
REFERENCES
12.51
BASIC LINEAR DESIGN
SECTION 12.3: GROUNDING
12.53
STAR GROUND
12.54
SEPARATE ANALOG AND DIGITAL GROUNDS
12.55
GROUND PLANES
12.56
GROUNDING AND DECOUPLING MIXED SIGNALS ICs WITH
LOW DIGITAL CONTENT
12.60
TREAT THE ADC DIGITAL OUTPUTS WITH CARE
12.62
SAMPLING CLOCK CONSIDERATIONS
12.64
THE ORIGINS OF THE CONFUSION ABOUT MIXED SIGNAL
GROUNDING
12.66
SUMMARY: GROUNDING MIXED SIGNAL DEVICES WITH LOW
DIGITAL CURRENTS IN A MULTICARD SYSTEM
12.67
SUMMARY: GROUNDING MIXED SIGNAL DEVICES WITH
HIGH
DIGITAL CURRENTS IN A MULTICARD SYSTEM
12.68
GROUNDING DSPs WITH INTERNAL PHASE-LOCKED LOOPS
12.69
GROUNDING SUMMARY
12.70
GROUNDING FOR HIGH FREQUENCY OPERATION
12.70
BE CAREFUL WITH GROUND PLANE BREAKS
12.73
REFERENCES
12.75
SECTION 12.4: DECOUPLING
12.77
LOCAL HIGH FREQUENCY BYPASS / DECOUPLING
12.77
RINGING
12.80
REFERENCES
12.82
SECTION 12.5: THERMAL MANAGEMENT
12.83
THERMAL BASICS
12.83
HEAT SINKING
12.85
DATA CONVERTER THERMAL CONSIDERATIONS
12.90
REFERENCES
12.96
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P RINTER C IRCUIT B OARD I SSUES
I NTRODUCTION
CHAPTER 12: PRINTED CIRCUIT BOARD (PCB)
DESIGN ISSUES
Introduction
Printed circuit boards (PCBs) are by far the most common method of assembling modern
electronic circuits. Comprised of a sandwich of one or more insulating layers and one or
more copper layers which contain the signal traces and the powers and grounds, the
design of the layout of printed circuit boards can be as demanding as the design of the
electrical circuit.
Most modern systems consist of multilayer boards of anywhere up to eight layers (or
sometimes even more). Traditionally, components were mounted on the top layer in holes
which extended through all layers. These are referred as through hole components. More
recently, with the near universal adoption of surface mount components, you commonly
find components mounted on both the top and the bottom layers.
The design of the printed circuit board can be as important as the circuit design to the
overall performance of the final system. We shall discuss in this chapter the partitioning
of the circuitry, the problem of interconnecting traces, parasitic components, grounding
schemes, and decoupling. All of these are important in the success of a total design.
PCB effects that are harmful to precision circuit performance include leakage resistances,
IR voltage drops in trace foils, vias, and ground planes, the influence of stray capacitance,
and dielectric absorption (DA). In addition, the tendency of PCBs to absorb atmospheric
moisture ( hygroscopicity ) means that changes in humidity often cause the contributions
of some parasitic effects to vary from day to day.
In general, PCB effects can be divided into two broad categories—those that most
noticeably affect the static or dc operation of the circuit, and those that most noticeably
affect dynamic or ac circuit operation, especially at high frequencies.
Another very broad area of PCB design is the topic of grounding. Grounding is a problem
area in itself for all analog and mixed signal designs, and it can be said that simply
implementing a PCB based circuit doesn’t change the fact that proper techniques are
required. Fortunately, certain principles of quality grounding, namely the use of ground
planes, are intrinsic to the PCB environment. This factor is one of the more significant
advantages to PCB based analog designs, and appreciable discussion of this section is
focused on this issue.
Some other aspects of grounding that must be managed include the control of spurious
ground and signal return voltages that can degrade performance. These voltages can be
due to external signal coupling, common currents, or simply excessive IR drops in
ground conductors. Proper conductor routing and sizing, as well as differential signal
12-1
BASIC LINEAR DESIGN
handling and ground isolation techniques enables control of such parasitic voltages.
One final area of grounding to be discussed is grounding appropriate for a mixed-signal,
analog/digital environment. Indeed, the single issue of quality grounding can influence
the entire layout philosophy of a high performance mixed signal PCB design—as it well
should.
12.2
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P RINTED C IRCUIT B OARD I SSUES
P ARTITIONING
SECTION 1: PARTITIONING
Any subsystem or circuit layout operating at high frequency and/or high precision with
both analog and digital signals should like to have those signals physically separated as
much as possible to prevent crosstalk. This is typically difficult to accomplish in practice.
Crosstalk can be minimized by paying attention to the system layout and preventing
different signals from interfering with each other. High level analog signals should be
separated from low level analog signals, and both should be kept away from digital
signals. TTL and CMOS digital signals have high edge rates, implying frequency
components starting with the system clock and going up form there. And most logic
families are saturation logic, which has uneven current flow (high transient currents)
which can modulate the ground. We have seen elsewhere that in waveform sampling and
reconstruction systems the sampling clock (which is a digital signal) is as vulnerable to
noise as any analog signal. Noise on the sampling clock manifests itself as phase jitter,
which as we have seen in a previous section, translates directly to reduced SNR of the
sampled signal. If clock driver packages are used in clock distribution, only one
frequency clock should be passed through a single package. Sharing drivers between
clocks of different frequencies in the same package will produce excess jitter and
crosstalk and degrade performance.
The ground plane can act as a shield where sensitive signals cross. Figure 12.1 shows a
good layout for a data acquisition board where all sensitive areas are isolated from each
other and signal paths are kept as short as possible. While real life is rarely as simple as
this, the principle remains a valid one.
There are a number of important points to be considered when making signal and power
connections. First of all a connector is one of the few places in the system where all
signal conductors must run in parallel—it is therefore imperative to separate them with
ground pins (creating a Faraday shield) to reduce coupling between them.
Multiple ground pins are important for another reason: they keep down the ground
impedance at the junction between the board and the backplane. The contact resistance of
a single pin of a PCB connector is quite low (typically on the order of 10 mΩ) when the
board is new—as the board gets older the contact resistance is likely to rise, and the
board's performance may be compromised. It is therefore well worthwhile to allocate
extra PCB connector pins so that there are many ground connections (perhaps 30% to
40% of all the pins on the PCB connector should be ground pins). For similar reasons
there should be several pins for each power connection.
Manufacturers of high performance mixed-signal ICs, like Analog Devices, often offer
evaluation boards to assist customers in their initial evaluations and layout. ADC
evaluation boards generally contain an on-board low jitter sampling clock oscillator,
output registers, and appropriate power and signal connectors. They also may have
additional support circuitry such as the ADC input buffer amplifier and external
reference.
12-3

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